Crowded House races in Penna.

WASHINGTON - Pennsylvania's most competitive U.S. House primary - or at least the one with the most competitors - has been playing out in the north-central part of the state. Twelve candidates are running for the state's only open seat in the House.

But there are also other House races to watch leading up to the April 22 primary.

In northeast Pennsylvania, Republican businessmen Chris Hackett and Dan Meuser have already run TV ads against each other. They are vying for the opportunity to unseat Rep. Chris Carney, a freshman Democrat in a heavily Republican district.

In western Pennsylvania, Republican Reps. Phil English and Tim Murphy each have four Democrats competing for the chance to unseat them. They could be taking a cue from Rep. Jason Altmire, a freshman Democrat also from western Pennsylvania who seemed to come from nowhere in 2006 to beat three-term Republican Rep. Melissa Hart.

Altmire was one of four House freshmen Democrats to unseat a Republican in the 2006 election. The wins tipped membership of Pennsylvania's 19-member House delegation in the Democrats' favor for the first time since 1998.

Nine Republicans and three Democrats are competing for the seat held by Rep. John Peterson, a Republican retiring at the end of his term. The district, which includes a wide swath of central and northern Pennsylvania, includes State College but is predominantly rural. Peterson has not endorsed a candidate, and there is no clear front-runner. But the winner of the Republican primary would likely have an advantage in November in the heavily Republican district.

The GOP candidates are Lou Radkowski, the Elk County coroner; Jeffrey J. Stroehmann, a business owner who is the great-grandson of the founder of Stroehmann Bakery; Derek Walker, owner of a financial planning business; Chris Exarchos, a former Centre County commissioner; Matt Shaner, a hotel developer; Glenn Thompson, the Centre County GOP chairman; Keith Richardson, an attorney and pastor; John Rea Stroup, mayor of Clarion; and John Krupa, an insurance agent.

The Democratic candidates are Bill Cahir, a journalist who fought in Iraq with the Marines; Mark McCracken, a Clearfield County commissioner; and Richard Vilello, mayor of Lock Haven.

The four candidates competing in the Democratic primary to challenge Murphy are Wayne Dudding, a colonel in the Army Reserve and an Iraq war veteran; Steve O'Donnell, a businessman; Beth Hafer, daughter of former state Treasurer Barbara Hafer and an executive in her mother's consulting firm; and Brien Wall, a life insurance company employee.

Those vying in the Democratic primary to challenge English are Tom Myers, a lawyer; Kathy Dahlkemper, director of the Lake Erie Arboretum; Kyle Foust, an administrator at Mercyhurst College and a member of the Erie County Council; and Mike Waltner, a program coordinator at the Episcopal Cathedral of St. Paul in Erie.

The primary is just a precursor to what will likely be an even more competitive election cycle in November. Among the competitive races without contested primaries:

A rematch between Altmire and Hart.

In northeast Pennsylvania, a contest between Democratic Rep. Paul Kanjorski, a 12-term incumbent, and Hazleton mayor Lou Barletta, a Republican who gained national attention for his efforts to crack down on illegal immigration.

In the Philadelphia region, a matchup between Rep. Patrick Murphy, a freshman Democrat and Iraq war veteran, and Republican Tom Manion, a retired Marine Reserve colonel and pharmaceutical executive whose son died last year in Iraq.